The myoseptal system in Chimaera monstrosa: collagenous fiber architecture and its evolution in the gnathostome stem lineage.

Recent studies have revealed the 3D morphology and collagen fiber architecture of myosepta in teleostome fishes. Here we present the first data set on the myoseptal structure of a representative of the chondrichthyan clade. We investigate the series of myosepta in the ratfish Chimaera monstrosa (Holocephali) from the anterior to the posterior body using microdissections of cleared and stained specimens, polarized light microscopy of excised myosepta, and histology. The features of the myoseptal system of Chimaera are compared to data from closely related vertebrate groups and are mapped onto a phylogenetic tree to further clarify the characteristics of the myoseptal series in the gnathostome ancestor. The 3D morphology and collagen fiber architecture of the myoseptal series in C. monstrosa resembles that of Teleostomi (Actinopterygii+Sarcopterygii) with regard to several features. Our comparative analysis reveals that some of them have evolved in the gnathostome stem lineage. (1) A series of epineural and epaxial lateral tendons (LTs) along the whole body, and a series of epipleural and hypaxial LTs in the postanal region evolved in the gnathostome stem lineage. (2) The LTs increase in length towards the posterior body (three-fold in Chimaera). Data on Chimaera and some comparative data on actinopterygian fishes indicate that LTs also increase in thickness towards the posterior body, but further data are necessary to test whether this holds true generally. (3) Another conspicuous apomorphic gnathostome feature is represented by multi-layer structures of myosepta. These are formed along the vertebral column by converging medial regions of successive sloping parts of myosepta. (4) The dorsalmost and ventralmost flanking parts of myosepta bear a set of mediolaterally oriented collagen fibers that are present in all gnathostomes but are lacking in outgroups. Preanal hypaxial myosepta are clearly different from epaxial myosepta and postanal hypaxial myosepta in terms of their collagen fiber architecture. In Chimaera, preanal hypaxial myosepta consist of an array of mediolaterally oriented collagen fibers closely resembling the condition in other gnathostome groups and in petromyzontids. Only one series of tendons, the myorhabdoid tendons of the flanking parts of myosepta, have evolved in the stem lineage of Myopterygii (Gnathostomata+Petromyzontida). Similar to LTs, the tendons of this series also increase in length towards the posterior body. In combination with other studies, the present study provides a framework for the design of morphologically based experiments and modeling to further address the function of myosepta and myoseptal tendons in gnathostomes.